Batteries Not Included

When you can get a D cell a few times cheaper than a 9V cell from a retail store, you know they don’t exactly price the batteries based on the cost of chemical goo inside, but rather by how often you need to buy them.

Here is my LC meter that I have built based on the design from here: https://sites.google.com/site/vk3bhr/home/lcm1

I don’t have a lot to say about the design as there are literally a dozen variation of the design on the web already.  All I did was to added a of my personal touches to it – an integrated AA battery holder, a DC/DC converter from an old TI calculator (a relabeled TL496).  I have also redesigned the “Zero” button to send a short reset pulse so that it no longer shows a corrupted display while you hold down the button.

LC meter

Here is a closer look at my battery holder. I build it by soldering 6 piece of copper clad PCB onto my main one.  The two end pieces are very strong “I” shaped structure with solder joins on each of its edges.  If you ever have to deal with odd sized batteries such as 1/3 AA, 18650 or cell phone batteries etc., this is one way to make a very sturdy battery mount for it.

The negative contact is made by a very springy Beryllium Copper alloy cut from a strip of EMI Spring Fingers.

PCB Battery holder

A closer view of the front shows an upside down ‘U” retainer for the positive terminal to secure the battery and gold-plated electrical contact made from a piece a flatten connector.

Battery holder at an angle

A form-fitting piece of those (vacuum formed) plastic packaging material fastened with double side tape is used to protect the LCD.

LC meter - front

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Neat tricks for the old resistor packs

Array chip resistors are handy to have.  You can make a voltage divider if you have some to spare on a PCB.   While the tolerances are no better than your regular resistors, their ratios are more closely maintained over temperature as they are side by on the same substrate.

2:1 divider – 2 resistors from the pack

3:1 divider – 3 resistors from the pack

4:1 divider – 4 resistors from the pack

5:1 divider – 4 resistors from the pack.  2 in series and 2 in parallel:  0.5/(0.5 + 2) = 1/5

resistor pack

If you add a jumper block to change the connections, you can change the voltage divider ratios.  It is pretty handy to add that to change the input range of you ADC on your microcontroller.

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